Cardiac stimulation devices deliver appropriately timed electrical stimulation pulses to a patient's heart to maintain a normal heart rhythm or improve synchronization of heart chambers. Patients having bradycardia, abnormalities of the heart's natural conduction system, a propensity for arrhythmias, cardiac-related breathing disorders, hemodynamic insufficiency, or other cardiac-related conditions may benefit from cardiac pacing therapies delivered in one or more heart chambers.
In order to effectively pace the heart, an electrical impulse delivered to the heart must have sufficient energy to depolarize the myocardial cells. Depolarization of the myocardial cells in response to a pacing pulse is often referred to as “capture.” The cardiac electrogram signal evidencing capture, which is a P-wave in the atria or an R-wave in the ventricles, is generally referred to as an “evoked response.” The lowest pacing pulse energy that captures the heart may be referred to as the “pacing threshold” or “capture threshold”. The amplitude and duration of a pacing pulse are preferably set to produce a pacing pulse energy somewhat greater than the pacing threshold in order to ensure effective cardiac pacing.
However, in order to prolong the battery life of the implanted cardiac stimulation device, it is desirable to program the pacing pulse energy to be a minimum value that is considered safely above the pacing threshold. Therefore, the pacing pulse amplitude is commonly set equal to the measured pacing threshold multiplied by a “safety factor.” The resulting pacing pulse amplitude setting provides a safety margin that ensures capture despite small fluctuations that may occur in the pacing threshold.
Pacing threshold can change over time due to tissue encapsulation of the pacing electrodes, lead movement, changes in the patient's clinical condition, changes in medical therapy, or other causes. A rise in pacing threshold can result in loss of capture and ineffective pacing therapy. Modern pacemakers typically include automatic pacing threshold search algorithms that automatically adjust the pacing pulse energy to ensure pacing pulses remain above the pacing threshold, even if it varies over time. A pacing threshold search may deliver pacing pulses starting at an initially high pulse energy that is greater than the pacing threshold and then progressively decrease the pulse energy until capture is lost. The lowest pulse energy at which capture still occurs is determined as the pacing threshold.
Thus, capture management systems typically include monitoring for changes in the pacing threshold and monitoring for evidence of capture during pacing. Capture monitoring may be performed continuously or periodically and typically involves sensing an evoked P-wave or R-wave following pacing pulse delivery. If loss of capture is detected, a pacing threshold search is performed and a new pacing pulse energy is set based on the pacing threshold search result and the programmed safety factor.
Situations may arise, however, when the pacing threshold is likely to be increased yet a pacing threshold search either fails or yields unreliable results. For example, during and after an episode of atrial fibrillation (AF) or atrial flutter (AFL), the atrial pacing threshold is typically increased. A pacing threshold search may be unsuccessful due to unstable thresholds and/or sensing. In another example, if a lead has shifted or become dislodged, the pacing threshold is likely to be increased, but a pacing threshold search may be unsuccessful due to sensing characteristics that have also changed. The programmed safety factor used in setting the pacing pulse energy may not be sufficient to ensure capture during these situations.
The safety factor is typically a fixed value that is programmable by the clinician. A dynamically variable safety margin is proposed in U.S. Pat. No. 6,456,882, issued to Schloss. An automatic capture/threshold capability is generally disclosed wherein the safety margin is periodically increased or decreased according to the performance of the stimulation device, i.e., based upon the frequency of capture. In U.S. Pat. No. 6,456,879, issued to Weinberg, a method is generally disclosed for altering stimulation energy based on rheobase and/or chronaxie shift of a strength-duration curve. The strength-duration curve may be divided into two regions having differently sized safety margins.
There remains a need, however, for recognizing situations in which a rise in pacing threshold can be expected yet a pacing threshold search may not yield a result for appropriately adjusting the pacing pulse energy.